1. Field of the Invention
The present invention relates to an image forming apparatus for detecting and correcting a skew without degrading a printing quality of an image recording medium, and in particular to a technique for determining a position by detecting a skew.
2. Description of the Related Art
Conventionally, an image forming apparatus of an inkjet system forms an image by causing a large number of nozzles of a recording head to eject ink droplets onto a recording medium. Particularly, if a mutual displacement of each color is large, a degradation of printing quality results in the case of obtaining a color image by ejecting plural colors of ink onto a recording medium in sequence.
For instance, in the case of an image forming apparatus using a fixed line head, if a skew, i.e., the transport direction of a recording medium skews vis-à-vis a nozzle array placed in the line head, a plurality of colors does not land in the same position of the recording medium, resulting in a color shift.
A laid-open Japanese patent application publication No. 10-35021 has proposed a method for correcting a color shift caused by such a skew. As shown by FIG. 1, an image forming apparatus comprises two recording heads 231 and 232 which are placed on the upstream side and one recording head 234 which is placed on the downstream side. The upstream recording head groups 231 and 232 are fixed onto a support member 233, while the downstream recording head group 234 is fixed onto a support member 235 which is equipped movably against a support member 233.
The support member 233 is equipped movably relative to a swing member 239 which is equipped swingably around a shaft 2310. And a detection unit 237 is equipped in the upstream of the recording heads 231 and 232 for detecting a registration mark 236 printed on both sides of a recording material 2311. The registration mark is printed along with the image by the each color head.
A calculation unit 238 detects a skew of the recording material 2311 based on information from the detection unit 237 which has detected the registration mark and controls the positions of the support members 233 and 235, and the orientation of the swing member 239, thereby preventing a shift of each color.
The detection of a skew by the method noted by the above described patent document prints registration marks on both sides of a recording medium. This requires a printing of inherently unnecessary registration marks, resulting in the unnecessary marks remaining until the both sides are cut off after printing the entire page, et cetera, otherwise degrading a print quality. Such is the problem.